1. Field of the Invention
This invention relates to a self-contained ground fault circuit interrupters and more particularly to a ground fault circuit interrupter that can be placed midspan between supply line conductors and load conductors and which contains within a single housing the devices and circuitry necessary for detecting a fault and a tripping relay responsive to the detection of a fault to open its contacts and terminate the flow of current to a load.
2. Description of the Prior Art
There has been a great need for means for detecting when an abnormal current is flowing through line to ground and for immediately interrupting the fault to halt such an abnormal flow to protect people from electric shock, fire, and explosion. As known in the prior art, the "differential" circuit breakers previously utilized in certain European countries have been generally unsatisfactory for such purposes because they have been too insensitive to ensure complete protection to human life. The prior art attempts to solve the aforementioned problem by providing a differential circuit breaker whose current interrupting contacts, in the event of a line to ground short circuit or an abnormal leakage current to ground, are operated by a semiconductor device which in turn is energized by the secondary of a differential transformer through whose core two conductors of the electrical circuit being monitored pass to effectively function as primary windings for the differential transformer.
Known is a ground fault circuit interrupter with an inadvertent ground sensor wherein a circuit breaker connected between a power source having a neutral conductor and a phase conductor and a load is operated when the differential transformer senses that more current is flowing into the load from the source through the conductors than is flowing back to the source through the conductors. A power transformer is connected across the neutral conductor and a phase conductor and has in its magnetic field a winding for inducing a small voltage between the neutral conductor and ground to sense an inadvertent grounding of the neutral conductor at or near the load. A tertiary winding of the power transformer is connected into the neutral conductor in the vicinity of the load whereby, in the event of a grounding of the neutral conductor in the vicinity of the load, a current is thus induced in the neutral conductor which passes into the ground in the vicinity of the load, and then into the ground for the neutral connector at the power line side of the differential transformer whereupon it passes through the primary of the differential transformer and, if large enough, causes the circuit breaker to open.
Also known is a ground fault protective system comprising a differential transformer having a toroidal core through which each of two line conductors and a neutral conductor pass to form primary windings of at least one turn. The secondary winding of the transformer serves as an output winding and is connected to a ground fault interrupter circuit which energizes the trip coil of a circuit breaker having a plurality of contacts connected to the conductors of the distribution circuit. The protective system further includes pulse generator means coupled to the neutral conductor for producing a high frequency current therein upon grounding of the neutral conductor between the differential transformer and the load. The high frequency current is produced by the periodic firing of a diac when the voltage on a capacitor connected thereto reaches a certain level. Thus, a continuous train of voltage pulses is applied to a winding of an output transformer and these pulses induce voltage pulses in the neutral conductor which passes through the transformer core. The voltage pulses induced on the neutral conductor have no effect upon the current balance in the distribution system as long as the neutral conductor is not grounded on the load side of the transformer. When such grounding does occur, the voltage pulses produce a current in the neutral conductor which does not appear in either of the line conductors. This imbalance is detected by the ground fault sensing means and causes the contacts to open, interrupting the flow of current in the distribution system.
Another known arrangement discloses an electrical circuit breaker including highly sensitive ground fault responsive means for protecting human life from electrical shock. Reference is made to the fact that prior art electric circuit breakers were not suitable for protecting human life which protection requires the detection of fault currents on the order of 3 to 50 milliamperes with load currents ranging on the order of 10 to 100 amperes. Sensitivity adequate to protect against ground faults is achieved by a circuit breaker comprising highly sensitive ground fault responsive means including a differential transformer having a toroidal core fabricated of a magnetic material. A line conductor and a neutral conductor pass through the opening in the toroidal core, forming single turn primary windings. The differential transformer also includes a secondary winding comprising a plurality of turns wound on the toroidal core. This secondary winding is connected to the remainder of the ground fault responsive means which includes a solenoid assembly comprising an armature, an operating coil, and a frame mounted on a casing. The armature is adapted for movement between an extended position and a retracted position in response to energization of the operating coil. A latch hook is attached to the armature and disposed for engaging the armature member of the actuator assembly. Thus, energization of the operating coil causes the latch hook to draw the armature away from a latch member to initiate tripping of the circuit breaker. The highly sensitive ground fault responsive means of this arrangement comprising the aforementioned solenoid assembly is capable of opening the circuit breaker contacts in response to ground fault current on the order of 3 to 5 amperes, and thus is desirable from the standpoint of protecting human life against electrical shock.
Yet another ground fault circuit interrupter comprises a differential transformer connected to an AC source which produces a voltage output when an imbalance in current flow between the power lines connected to the AC source occurs. This AC signal voltage is coupled to a differential amplifier through a coupling capacitor, rectified, current limited, and applied to a gate of an SCR. When the SCR conducts, the winding of a transformer connector across the power line is energized, causing two circuit breaker switches to open. Also provided is a ground fault circuit for closing the switch when the line becomes unbalanced.
Still another known arrangement uses a ground leakage protector including a ground fault release coil controlled by a ground fault detector. The ground fault release coil is normally energized, and is deenergized when a ground fault appears which disables a restraining latch which results in the opening of the circuit breaker.
Yet another known arrangement uses a unitary circuit breaker of the molded case type including, within its casing means sensitive to ground faults, means sensitive to overcurrents, and means sensitive to short circuit currents, all of which act on a common trip latch of the breaker to cause automatic opening. The ground fault sensitive means comprises a current imbalance detecting coil which energized a tripping solenoid, releasing a normally latched plunger to cause tripping.
Also known is a ground fault protection system that employs a dormant oscillator which is triggered into oscillation to initiate disconnection of the protected distribution circuit upon occurrence of a neutral to ground type of fault.
None of the aforementioned prior art suggests a tripping relay which is not self-contained but rather is mounted in a circuit interrupter.